1. Field of the Invention
The present invention is directed to wet air oxidation systems and methods for providing an oxidized waste stream substantially free of volatile compounds and, in particular, an oxidized waste stream substantially free of volatile organic compounds and/or ammonia.
2. Description of the Related Art
Wet air oxidation methods are a preferred method for treating waste streams containing oxidizable constituents, especially volatile organic compounds (VOCs), which are relatively easily oxidized. Moreover, the reaction products of the oxidation of organic compounds, including VOCs, are carbon dioxide and water, which generally do not present disposal or discharge problems. The oxidizable constituents of the waste stream may also include inorganic compounds that are soluble or suspended in the waste stream, such as, for example, reduced sulfur compounds and/or cyanides. Typical operating conditions at which most wet air oxidation units are operated are a temperature of about 150.degree. C. to about 320.degree. C. and a pressure of about 50 psig to about 4000 psig. Regardless of the operating temperature, since wet air oxidation reactions take place in a waste stream which comprises water, a superatmospheric pressure must be used during the wet air oxidation in order to prevent the water from evaporating at the typical operating temperatures. Consequently, the volatility of many compounds is suppressed during wet air oxidation due to the superatmospheric pressures. Therefore, at typical operating conditions, varying levels of volatile compounds may remain solubilized or suspended in the oxidized effluent, causing disposal and discharge problems. For example, many oxidized liquid effluents from wet air oxidation processes include constituents that may be toxic, even at relatively low concentrations, to the biological organisms found in a biological treatment plant. State or federal environmental regulations may also regulate the discharge of constituents that may be contained in such oxidized liquid effluents.
During wet air oxidation, there are competing pathways for the removal of volatile compounds in a wet air oxidation system. For example, volatile compounds in a waste stream may either be oxidized, or volatilized into a gas phase. The volatility of a volatile compound depends on the temperature and pressure at which the wet air oxidation system is operated. In general, increased temperatures tend to increase both volatility and the rate of oxidation. Increased pressure tends to suppress volatility, but does not substantially affect the rate of oxidation. Therefore, as operating temperatures in a wet air oxidation system increase, oxidation predominates, whereas at relatively lower operating temperatures, volatilization predominates.
Organic compounds may additionally contain other atomic species including, for example, sulfur, phosphorus, nitrogen, and halogens. During the wet air oxidation of such organic compounds, organically bound sulfur will be converted to inorganic sulfate; organically bound phosphorus will be converted to inorganic ortho phosphate; organically bound halogens will be converted to the corresponding inorganic halide; and organically bound nitrogen will be converted to ammonia nitrate, nitrate, nitrous oxide or molecular nitrogen.
At room temperature and ambient pressure, ammonia is a gas having a relatively high partial pressure that would normally allow it to volatilize. At the operating temperatures at which most wet air oxidation units are operated (typically about 150.degree. C. to about 320.degree. C.), ammonia is relatively stable. Generally, ammonia requires higher operating temperatures to undergo an oxidation reaction (typically greater than 320.degree. C.). However, the volatility of compounds that are volatile at atmospheric pressure, including ammonia, are suppressed in wet air oxidation processes due to the superatmospheric pressure required to prevent water from evaporating.
Ammonia also has a characteristic tendency to act as a base, reacting with acids to produce the ammonium ion. Carbon dioxide, which is a typical reaction product of wet air oxidation, is an acid which imparts acidic behavior to the oxidation environment. Most wet air oxidation reactions are conducted in an acidic environment, which, in water, implies an excess of hydrogen ions (H.sup.+). Under such conditions, ammonia will react with hydrogen ions to yield an ammonium ion (NH.sub.4.sup.+) as follows: EQU NH.sub.3 +H.sup.+ .fwdarw.NH.sub.4.sup.+
Once ammonia is converted to the ammonium ion, it can no longer be volatilized under typical operating conditions. Consequently, the removal of ammonia from a waste stream is particularly difficult to achieve with conventional wet air oxidation systems since ammonia is not oxidized, its volatility is suppressed, and it tends to be converted to ammonium ion, which is retained in the wet air oxidized effluent. Ammonia, in particular, is a concern for biological treatment systems due to its potential for biological toxicity or inhibition.
Accordingly, a system and method for providing an oxidized waste stream substantially free of volatile compounds, particularly VOCs and/or ammonia, is desirable.